As a method of evaluating the performance of a seat, there are performed pressing a predetermined portion by a pressure board, obtaining a relationship between force (load) and a displacement (bending) at this moment, and thereby obtaining a spring constant (static spring constant) as an elasticity characteristic of the portion, or there are performed mounting a weight on a predetermined portion, applying vibration in a state of being stable by its own weight, drawing on a Lissajous figure a relationship between a load working on the weight (F=ma (m is mass of the weight, a is acceleration generated on the weight by vibration) and a relative displacement of the weight, and obtaining the static spring constant from inclination in the Lissajous figure. On the other hand, for evaluating vibration damping performance, there is performed obtaining a spring constant (dynamic spring constant) as a dynamic elasticity characteristic from the static spring force and damping force.
When evaluating seating performance, it is preferable to use a pressure board or weight that is close to a human model, and it is based on evaluation using a pressure board or weight with the size corresponding to the range of a diameter of approximately 100 mm (diameter of 98 mm precisely) around a portion below the ischial tuberosities to be the apex of a body pressure distribution, that is, the size corresponding to one of the buttocks or one of the femurs of a human. Further, the mass of a weight when measuring the dynamic spring constant is based on a mass corresponding to pressure in the range of a diameter of approximately 100 mm (diameter of 98 mm precisely) around the portion below the ischial tuberosities.
Then, when evaluating a seat on the assumption that the feeling of support when a person is seated on the seat cushion is determined by whether sufficient support can be obtained or not in the portion below the ischial tuberosities, evaluation using the pressure board or weight with the diameter of 98 mm is regarded as important for both the static spring constant and the dynamic spring constant, and in the evaluation data, both the static spring constant and the dynamic spring constant are typically set to be highest in the portion located below the ischial tuberosities and to decrease toward a front side of the seat cushion, such as decreasing in order of the portion corresponding to the vicinities of femoral bases and the portion corresponding from the vicinities of substantially centers of the femurs to the vicinities of the backs of knees. Specifically, seat designing is based on increasing the supportability for a human body by setting to high values the static spring constant and the dynamic spring constant below the ischial tuberosities corresponding to the barycentric position of a human body. However, since the seat having such a structure has a large dynamic spring constant below the ischial tuberosities, it has a drawback in that the influence of vibration inputted via members (cushion member, frame member, and the like) arranged below the ischial tuberosities is large.
In view of such point, the present inventor has suggested in Patent document 1 a seat in which spring members having different spring constants are coupled in series to thereby make the static spring constant of a human body support portion (spring element) below the ischial tuberosities to be small, and thereby make the dynamic spring constant to be small as well. According to this structure, the influence of vibration transmitted via the members arranged below the ischial tuberosities becomes small.
Patent document 1: Japanese Patent Application Laid-open No. 2005-7078